The major purposes of this research are (1) to explore the extent, the nature, and the clinical importance of heterogeneity of erythrocyte NADH methemoglobin reductase, and (2) to more fully analyze the methemoglobin-reducing process in human erythrocytes and the physiology of the NADH methemoglobin reductase system. The methods employed include electrophoresis in starch and polyacrylamide gels; chromatographic separations using Sephadex, agarose, and calcium phosphate gels, and a variety of ion-exchange resins; spectrophotometric assay of enzymes; fractionation of erythrocytes according to their density; immunization of chickens with purified enzymes; and the detection of raised antibodies utilizing immunodiffusion and neutralization techniques.